1. Field of the Invention
The present invention is directed to the reduction of distortion that is added to a signal by a nonlinear device (e.g., power amplifier, mixer) in a transmitter.
2. Description of Related Art
Transmitters use complex modulation schemes (e.g., 16QAM, 64 QAM, 32TCM, 128TCM) to make efficient use of the available spectrum. However, these complex modulation schemes are very sensitive to all types of distortion and, in particular, to nonlinear types of distortion caused by nonlinear devices (e.g., power amplifiers, mixer) in the transmission chain. Therefore, it is important to reduce the distortion added to a signal by nonlinear devices so that transmitters can use complex modulation schemes and make efficient use of the available spectrum.
It is well known that power amplifiers have nonlinear characteristics, see FIG. 1 which illustrates a PowerIN-PowerOUT graph 100 that shows a linear zone 102 and a saturation zone 104 of a power amplifier. As shown, if the power amplifier is used in the linear zone 102, then the full power of the power amplifier may not be utilized. And, if the power amplifier is used in the saturation zone 104, then the power amplifier unacceptably distorts an input signal. As such, the power amplifier should operate near but not to close to the saturation zone 104. The efficiency of the power amplifier can be increased by extending the linear range of the linear zone 102 into and above the saturation zone 104 (see dashed line). A nonlinear correction device known as a linearizer can be used to increase the efficiency of the power amplifier. There are many different types of linearizers used today including feedback linearizers and feedforward linearizers. Examples of a traditional feedback linearizer and a traditional feedforward linearizer are briefly discussed below with respect to FIGS. 2 and 3.
Referring to FIG. 2 (PRIOR ART), there is shown a block diagram of a transmitter 200 incorporating a traditional feedback linearizer 202. Certain details associated with the transmitter 200 such as a modulator, filter and mixer are well known in the industry and as such need not be described herein. Therefore, for clarity, the description provided below in relation to the transmitter 200 omits the components not necessary to understand the invention.
The transmitter 200 receives an undistorted input signal 204 at an input terminal 206 that is processed by a subtractor 208 and amplified to a higher power level by a power amplifier 210 which generates an output signal 212 that is transmitted by an antenna 214. As described above, the power amplifier 210 imparts some distortion (e.g., nonlinear components, nonlinear spurs) to the input signal 204 which without the presence of the feedback linearizer 202 would be present in the output signal 212. To help compensate for the distortion, a portion of the output signal 212 is applied to a voltage divider 216 which outputs a lower powered signal 216. The lower powered signal 218 is applied to the subtractor 208. In the subtractor 208, the lower powered signal 218 which is 180xc2x0 out of phase with the input signal 204 is added to the input signal 204 which helps to compensate for the nonlinearities that are going to be added by the power amplifier 210 to the input signal 204. As such, the feedback linearizer 202 including the subtractor 208 and the voltage divider 216 helps to compensate for at least some of the distortion that is going to be added to the input signal 204 by the power amplifier 210. Essentially, the traditional feedback linearizer 202 relies on a feedback loop to compensate for the nonlinearities produced by the power amplifier 210.
A main drawback of the traditional feedback linearizer 202 is that only a limited amount of correction can be achieved with this arrangement. Because, the delay associated with the feedback loop limits the effectiveness of the feedback linearizer 202. In practice, the traditional feedback linearizer 202 is difficult to implement and often become unstable in transmitters that operate at higher frequencies (e.g., 2 GHz-11 GHz).
Referring to FIG. 3 (PRIOR ART), there is shown a block diagram of a transmitter 300 incorporating a traditional feedforward linearizer 302. Again, certain details associated with the transmitter 300 such as a modulator, filter and mixer are well known in the industry and as such need not be described herein. Therefore, for clarity, the description provided below in relation to the transmitter 300 omits the components not necessary to understand the invention.
The transmitter 300 receives an undistorted input signal 304 (see exploded view where signal 304 includes two tones) at an input terminal 306 which is split by a splitter 308 and input into a power amplifier 310 and a time delay circuit 312. The power amplifier 310 amplifies one portion of the input signal 304 and outputs a distorted signal 314. The time delay circuit 312 delays the other portion of the input signal 304 and outputs a delayed signal 316. The delayed signal 316 has the same phase as the distorted signal 314. The distorted signal 314 is then stepped downed and forwarded by a coupler 318 to an input of a subtractor 320 that also receives the delayed signal 316 and outputs a nonlinear signal 322. In effect, the subtractor 320 cancels the main signals 324 of the distorted signal 314 and the delayed signal 316 such that only the nonlinear spurs 326 remain which form the nonlinear signal 322. The nonlinear signal 322 is amplified by an error power amplifier 328 which outputs a distorted nonlinear signal 330. It should be noted that the error power amplifier 328 has the same characteristics as power amplifier 310. At the same time, another time delay circuit 332 delays the distorted signal 314 and outputs a delayed distorted signal 334. The delayed distorted signal 334 has the same phase as the distorted nonlinear signal 330. The nonlinear distorted signal 330 is then stepped up and forwarded by a coupler 336 to an input of another subtractor 338 that also receives the delayed distorted signal 334 and outputs a compensated signal 340. The compensated signal 340 has the same main signals 324 and smaller nonlinear spurs 326 when compared to the distorted signal 314. The compensated signal 340 is transmitted by an antenna 342.
A main drawback of the traditional feedforward linearizer 302 is that it is a complex system in that it is difficult to tune the time delay circuits 312 and 332, the couplers 318 and 336, the power amplifiers 310 and 328 and the subtractors 320 and 338. Another drawback of the traditional feedforward linearizer 302 is that it is expensive to make and difficult to implement in transmitters that operate at higher frequencies (e.g., 2 GHz-11 GHz). Accordingly, there has been a need for a linearizer that can address the aforementioned problems and other problems associated with traditional linearizers. These needs and other needs are addressed by the predistortion linearizer and method of the present invention.
The present invention includes a predistortion linearizer, transmitter and method for linearizing a nonlinear device (e.g., power amplifier, mixer). Basically, the predistortion linearizer includes a coupling circuit, a diode and a direct current adjusting circuit that work together to generate a distorted signal which is reflected onto a signal path and inputted into the nonlinear device. The distorted signal compensates for at least some of the nonlinear spurs introduced by the nonlinear device to an input signal which was also applied to the signal path and inputted into the nonlinear device. As a result, the nonlinear device outputs a compensated output signal.